The present invention relates to a process for producing-ultrafine or xe2x80x9cnano-sizedxe2x80x9d titanium dioxide, parts of the process, and the product of the process. In particular, the present invention relates to the processing of aqueous titanium solutions to ultrafine titanium dioxide. The process includes a novel combination of operational steps to economically produce a high quality-ultrafine titanium dioxide.
Titanium dioxide (TiO2) for pigment use may be present in either of two predominant forms, anatase or rutile, and is used in paints, paper, plastics, ceramics, inks, etc. The titanium dioxide for pigment use generally has an average particle size of 150 to 250 nanometer and is considered the principal white pigment of commerce. It has an exceptionally high refractive index, negligible color, and is quite inert. Titanium dioxide having a smaller average particle size, for instance in the 10 to 100 nanometer median particle size range, is used commercially in cosmetics and personal care products, plastics, surface coatings, self-cleaning surfaces, and photovoltaic applications. This titanium dioxide is referred to as ultrafine or nano-sized titanium dioxide.
There are several processes for making ultrafine titanium dioxide, some in commercial use and some in development. Some use anhydrous titanium tetrachloride as a feedstock and burn it in an oxygen-hydrogen flame or in a plasma arc. Another process uses a titanyl sulfate solution as the feedstock. In this process, titanium dioxide is precipitated from the feedstock in a controlled manner, followed sequentially by calcination and intense steam micronization to break up agglomerates formed during the calcination step.
Both types of processes, however, suffer from a lack of control over the product particle size distribution, as well as the product mineralogy. In other words, although the average particle size of the titanium dioxide may be from 10 to 100 nanometer, the particle size distribution may vary greatly. Furthermore, the titanyl sulfate process produces an anatase form whereas the anhydrous chloride oxidation produces a rutile crystallographic modification. Therefore, there is a need for a process to economically make ultrafine titanium dioxide having a narrow particle size distribution and having a specific crystallographic modification made by an easily controlled, lower cost, environmentally sound route.
The present invention provides an economical hydrometallurgical process for producing ultrafine TiO2 from aqueous titanium chloride solutions. The solutions may be derived from any of several sources, but are practically derived from processing mineral ores and, in particular, ilmenite ore or an ilmenite mineral concentrate. The processing to produce the solutions can be, for instance, a leaching or dissolution process, followed by any of several means for solution purification. The solution could also be produced by hydration of anhydrous titanium tetrachloride. The solution may also be derived from the process described in application No. 60/141,114 and from U.S. application Ser. No. 09/500,207, the entire contents of which are incorporated herein by reference.
The aqueous titanium chloride solution is generally comprised of water, hydrochloric acid, titanium oxychlorides and titanium chlorides. The solutions may vary widely in composition with respect to the hydrochloric acid content and the titanium content.
Minor quantities of any of several chemical control agents may be introduced into the solution. The solution is further converted to a titanium oxide solid in a process involving total, controlled evaporation of the solution and the formation of a thin film of titanium dioxide. This process is conducted above the boiling point of the solution and below the temperature where there is significant crystal growth. The water and hydrochloric acid are vaporized and the hydrochloric acid may be recovered.
The titanium oxide is next calcined at an elevated temperature to induce and control crystallization. The concentration and type of chemical control agent as well as the calcination conditions determine the desired crystalline form and crystal size of the ultrafine titanium dioxide.
Following calcination, the titanium dioxide is milled or dispersed to yield a final nano-sized or ultrafine titanium dioxide having a narrow particle size distribution.
The advantages of the process according to the present invention include a superior high quality ultrafine titanium dioxide due to the narrow particle size distribution and readily controlled physical and chemical characteristics, and low cost processing.